In the combustion of pulverised coal for steam generation in coal-fired power stations there are certain fixed losses determined for example, by plant design, and certain controllable losses caused by operating under non-ideal conditions. The controllable losses comprise:
(a) losses due to incomplete combustion of both solids and combustible gases; PA1 (b) losses due to the need for excess air.
In practice the controllable losses show a minimum as a function of oxygen in the flue gas and it is preferable to operate near this minimum. One way this can be achieved is by basing control of the boiler on the measurement of oxygen and carbon monoxide in flue gas. Most large boilers today are equipped with oxygen analysers which measure O.sub.2 at one point in a duct. A problem with these analysers is that the reading is drastically distorted by air infiltration into the furnace and in the convection passages downstream of the burners. Also, as measurements are made at one point, sampling errors are large.
Carbon monoxide in flue gas stays at very low levels at high excess air and rises as excess air is reduced. Infrared CO analysers are available which direct the IR beam across the stack, thus minimising sampling errors. However, optimising excess air using CO monitors generally produces a large amount of unburnt carbon in the ash, because CO levels are very low at optimum excess air.
An alternative technique is to base control of the boiler on the determination of unburnt carbon in the fly ash. A 500 MW power station burning black coal of 20% ash will produce about 2500 tonnes/hr flue gas, and 37 tonnes/hr fly ash. The carbon content of this fly ash will be normally in the range 2-5 wt % although it may contain up to 15 wt % carbon. Typically the fly ash concentration in flue gas is about 20 g/m.sup.3. Present instruments for the determination of the carbon content of the fly ash rely on extracting a sample, typically less than 1 gram, from the duct and analysing this on a batch basis typically at 10-20 minute intervals.
One prior art carbon concentration monitor [Rupprecht and Patashnick Co., Inc, NYSERDA Report 86-2, January 1986] is based on a microbalance and small furnace. The instrument collects a 10-50 mg sample of fly ash from the outlet duct of a boiler and determines the unburnt carbon in this sample from the mass loss after heating at 750.degree. C., this measurement cycle being repeated at approximately 15 minute intervals. One disadvantage of this analysis technique is that it is very difficult to collect a representative sample of such small size, and therefore sampling uncertainty significantly limits the accuracy of the unburnt carbon determination. The analysis accuracy for replicate samples in laboratory tests was approximately .+-.0.5 wt % at 2.3 wt % carbon.
Another commercially available device [Energy and Environmental Research Corporation, 18 Mason, Irvine, Calif., USA; December 1987] for the determination of unburnt carbon in fly ash collects an approximately 1 gram sample from the duct using an isokinetic sampler and analyses this for unburnt carbon content from the measured surface reflectance of the sample. The sample collection and measurement cycle is repeated at approximately 5 minute intervals. In a plant test of the instrument at the Nefo power plant, Denmark, the analysis accuracy was approximately .+-.1 wt % at less than 3 wt % carbon and .+-.0.5 wt % at greater than 3 wt % carbon. The analysis accuracy is limited by sampling uncertainty, due to the sample size and measuring principle (i.e. surface reflectance) used, and the sensitivity of the reflectance measurement to coal type.
A device based on a measurement of the capacitance of a fly ash filled capacitor has been proposed for the determination of carbon in fly ash in Australian Patent 562440. In this arrangement ash is taken from an ash hopper using a screw conveyor, fed into a measuring chamber into the electric field established by the electrodes of a capacitor and the change in capacitance of the capacitor measured, and finally returned to the ash hopper using a second screw conveyor. The bulk density of the ash in the measuring chamber is assumed to be approximately constant, although compensation for variation in the bulk density is possible using a weighing device.
A microwave technique has been proposed for simultaneously reducing and measuring the carbon content in fly ash in U.S. Pat. No. 4,705,409. In this technique ash is taken from an ash hopper and passed through a metallic waveguide. Microwave radiation directed through the guide is preferentially absorbed by the carbon in the fly ash, and the concentration of carbon is determined from measuring the temperature rise of a water wall surrounding the guide. Sufficient microwave power is injected into the guide to burn the excess carbon in the ash and generate a reduced carbon product. One disadvantage of this technique is that the heat conduction out of the guide, and the associated temperature rise in the water wall, is a function of not only the carbon content of the ash but also the chemical characteristics, temperature and heat conduction properties of the ash. These factors need to be taken into account in the calibration and operation of the device.
Nuclear measurement of carbon in fly ash has also been investigated [Steward, R. F., ISA Transactions, (3), 1967, 200-207]. In this technique carbon concentration is correlated with counts of 4.43 MeV gamma rays produced from carbon atoms by the inelastic scatter of neutrons. Using this technique in laboratory measurements on 10 kg fly ash samples the analysis accuracy is repeated as .+-.0.5 wt % over the range 2-16 wt % carbon.